Localized Spin Rotations: A Size-Consistent Approach to Nonorthogonal Configuration Interaction.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Current nonorthogonal configuration interaction (NOCI) methods rely on chemically intuitive selections of self-consistent field (SCF) states.
• Tracking these SCF states during molecular dissociation can be difficult, and resulting NOCI states may suffer from spin contamination.

Purpose of the Study:

• To develop an improved NOCI methodology that addresses challenges in state selection and spin contamination.
• To provide a robust and accurate computational approach for studying molecular dissociation and electronic structure.

Main Methods:

• A novel method applying spin rotation to symmetry broken unrestricted Hartree-Fock (sb-UHF) states to generate a basis for NOCI.
• Examination of ethene dissociation by localizing spin rotation on carbene fragments.
• Comparison with dissociation studies using canonical UHF and absolutely localized molecular orbitals (ALMO).

Main Results:

• The proposed method yields a size-consistent description of molecular dissociation.
• Spin-pure states are obtained across all geometries.
• The method successfully restores spin symmetry for symmetry-broken SCF wave functions in various molecules.

Conclusions:

• The developed spin rotation technique offers a significant improvement over existing NOCI methods.
• This approach provides accurate and spin-pure electronic structure descriptions, particularly for dissociating systems.
• The method's applicability to diverse molecular systems highlights its potential for broader use in computational chemistry.